A variety of controllers are used to control vehicle systems. One example of a vehicle-based controller is an inverter controller. The inverter controller is used to control the vehicle traction drive and numerous other vehicle systems. When using the inverter controller in vehicle systems, it is desirable to minimize current and torque pulsations, particularly at high speeds.
Discontinuous Pulse Width Modulation (DPWM) methods are commonly employed in inverter controllers to control the fundamental output voltage component of three-phase voltage source inverters. These three-phase voltage source inverters may in turn be used to control the phase currents of three-phase Alternating Current (AC) motors. DPWM methods reduce inverter losses in comparison to continuous Pulse Width Modulation (PWM) methods, such as sinusoidal or space vector modulation.
In general, a PWM signal has a modulation index that defines the amplitude of the fundamental output voltage component produced by the three-phase voltage source inverter. This modulation index is often defined in terms of a maximum fundamental output voltage that can be produced by the three-phase voltage source inverter. The modulation index (Mi) is given as:
      M    i    =            V      1      *                      2        π            ⁢              V        dc            where Vdc is the Direct Current (DC) voltage provided to the three-phase voltage source inverter and V1* is a commanded amplitude of the fundamental output voltage component.
Most PWM methods used with voltage source inverters are susceptible to voltage distortion due to practical limitations of the voltage source inverter, such as inverter lockout time and minimum pulse width constraints. These practical limitations are typically non-linear effects that manifest as finite and controllable minimum and maximum pulse widths. Either inverter switch, for a phase leg, of the voltage source inverter can be indefinitely held “ON” to create discrete values of pulse widths with duty cycles of zero and one, respectively. During some operating conditions, typically at high values of Mi, the commanded duty cycles for a particular phase leg have a pulse width between the minimum and maximum achievable pulse widths and the corresponding discrete values of zero and one. The non-linear effects produce unachievable regions that occur for each phase of the voltage source inverter (e.g., four times per fundamental cycle).
In the unachievable regions, the inverter control is typically set to clamp the duty cycles at either the maximum pulse width or to one of the voltage rails in a continuously “ON” condition. Analogously, the inverter control may also be set to clamp the duty cycle at either the minimum pulse width or to a lower voltage rail. Either of these conventional settings alters the output fundamental voltage component produced by the voltage source inverter, and the input-output voltage relationship of the modulation index (Mi) becomes non-linear.
Accordingly, it is desirable to provide a method of controlling a fundamental output voltage component of voltage source inverters that maintains an input-output voltage linearity relationship. Additionally, it is desirable to provide a controller that controls a fundamental output voltage component of a voltage source inverter while maintaining an input-output voltage linearity relationship. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.